Weathered crust karst-type oil-gas reservoirs are one of important types of carbonate oil-gas resources. A large number of studies have shown that a key factor of controlling development and distribution of weathered crust karst-type reservoirs lies in a development degree of karstification. Forming erosion fractured cavities by modification of the original formation by means of karstification, which has a significant impact on reservoir properties of reservoirs and enhancement of seepage capability. Therefore, how to accurately identify weathered crust karst-type reservoirs and classify the development degree of karstification by using the existing logging methods can provide important technical basis for distinguishing reservoir validity, increasing oil and gas reserves and developing measures for oil and gas wells.
An evaluation of a karst development degree in the prior art is mainly based on observation and description of cores, where a qualitative classification of dissolved apertures on the surface of rocks and developments of fractures is performed by coring the reservoirs or sampling and observing outcrop rock drilling. Such methods are highly dependent upon cores with high cost and poor operability. In addition, since the cores acquired in the oil field are always discontinuous, and a complete coring is very difficult at the fracture development interval, it is difficult for the karst development features observed on the cores to represent overall conditions of reservoirs, thereby affecting a comprehensive discrimination of the reservoirs.
Although imaging logging in conjunction with conventional logging techniques have been used in the prior art to classify weathered crust karst-type reservoirs into karst zones, such as a supergenic karst zone, a vertical vadose zone, a horizontal hyporheic zone and the like longitudinally, and identify the karst zones according to imaging logging feature pattern plates and logging response feature modes of different karst zones qualitatively. However, the development degree of specific karstification of the karst zones is not classified or evaluated, thus an application of processing interpretation of the imaging logging data essentially remains in the qualitative analysis stage, it cannot meet requirements of logging evaluation of the weathered crust karst-type reservoirs.